[40th Anniversary of PGF Celebration at Willow Creek, CA]

LOS ANGELES, United States (AFP) — Forty years after two cowboys filmed an unidentified creature ambling through a California forest, hunters of "Bigfoot" say the grainy footage remains the cornerstone of their belief in the legendary ape-like beast's existence.

The subject of numerous hoax theories and investigations, Roger Patterson and Tom Gimlin's film, shot at Willow Creek in the expanse of the Six Rivers National Forest on October 20, 1967, has been debunked, derided and dismissed.

But so far it has never been successfully duplicated, and it is that fact, that Bigfoot devotees say is crucial.

Daniel Perez, an avowed follower of the beast known as Sasquatch, describes the Patterson-Gimlin footage as the missing piece of the Bigfoot puzzle.

"This is the cornerstone of the entire case for Bigfoot," Perez told AFP. "People say 'It's just a guy in a suit'. Well I beg to differ. If it's a man in a costume, why can't anyone duplicate the film?"

Perez, who will address a conference of Bigfoot believers at Willow Creek on Saturday to mark the film's 40th anniversary, claims that enhancements of the original, jerky footage show the creature's muscles expanding, rippling and contracting from one frame to the next.

"You can't do that with a costume," Perez says. "A costume conceals rather than reveals."

Tom Yamarone, a member of the Bigfoot Field Researchers Organization, a US group of enthusiasts who record sightings and embark on hunts for the creature, echoed Perez's argument.

"The Patterson-Gimlin footage, despite it's shakiness, is what we would refer to as the gold standard for visual evidence," says Yamarone.

Yet others are not convinced. In his 2004 book "The Making of Bigfoot" author Greg Long claims to have identified the 'man in the suit' as a worker from Washington state, Bob Hieronimus.

Long also casts a critical eye over Patterson's role in the affair, noting the extraordinary coincidence that he had set out into the forest to shoot a documentary about Bigfoot and returned with footage of the creature

Robert Stein, an expert in trick photography, echoed Long's scepticism in an interview.

"Roger Patterson sets out to make a Bigfoot documentary," he says. "He immediately stumbled upon a Bigfoot. Not only that, he stumbled upon a Bigfoot that was out in the open in bright, clear sunshine, perfect weather for filming. He didn't just beat the odds. He gave the odds a royal whipping."

Patterson, who died of cancer in 1972, is portrayed in Long's book as a chronically unemployed rodeo rider determined to profit from an elaborate hoax.

Gimlin has always insisted the film was genuine, denying any knowledge of Long's claim that Patterson had in fact ordered a gorilla suit from a costume supplier.

And experts in special effects have questioned the 'man-in-a-suit' theory, saying the level of sophistication displayed in the film would likely have been beyond the reach of ordinary hoaxers.

Ryan Peterson, a freelance make-up effects artist who has worked on films such as "Mighty Joe Young" and "Men In Black," believes that if the film was a hoax, the perpetrators were operating years ahead of their time.

"In my opinion, the technology and artistry were not available in 1967 to create such a convincing Bigfoot costume," Peterson wrote in the Salt Lake Tribune this week.

Perez, a 44-year-old electrician who has been chronicling Bigfoot since childhood, says he is open to arguments that the film is a hoax.

"I would be willing to change my view on the subject of the film if they were able to duplicate it," he says. "But you expect me to believe that a broke cowboy and his friend Bob Gimlin make this sophisticated movie? Come on."